The present invention relates to a digital transmission system by using the orthogonal frequency division multiplex (hereinafter, abbreviated as OFDM), and more particularly to an OFDM demodulation method for correcting a DC offset generated in the OFDM demodulator and a semiconductor integrated circuit using the same method.
Recently, the OFDM transmission method prevails as the transmission method for realizing a broadband digital communication such as digital television broadcast and wireless LAN. This OFDM transmission method is one of the multi-carrier transmission methods and it is a transmission method for performing the digital modulation on a plurality of mutually orthogonal subcarriers, and therefore, a broadband digital communication can be realized. The OFDM transmission method, however, requires high accuracy in demodulation of subcarriers because the interval of the frequencies of subcarriers is comparatively narrow. Especially the DC offset generated in an analog unit such as an A/D converter of the OFDM demodulator has an adverse effect on the demodulation of subcarriers, which deteriorates the performance of the OFDM demodulation. Therefore, it is extremely important to correct the DC offset and reduce the adverse effect in order to improve the performance of the OFDM demodulator.
In the conventional OFDM demodulator for correcting the DC offset, an OFDM signal with the channel central frequency mainly modulated is received by an antenna and supplied to an RF unit. The RF unit amplifies the received signal, and a frequency converter converts the signal of the channel central frequency into intermediate frequency and further converts it into the I signal and Q signal that are baseband signals, through the I/Q separation. The I signal and the Q signal converted into the baseband signals are analog signals and the respective signals are converted into time discrete and amplitude discrete digital signals by the A/D converter. The digital baseband signals are entered into a DC offset estimating unit of a baseband unit. The DC offset estimating unit has a function of estimating the DC offset generated in the A/D converter. For example, according to the technique disclosed in Japanese Patent Laid-Open No. 6-326739, the average voltage during the period of nul symbol included in the OFDM baseband signal is measured, thereby estimating the DC offset. Here, the nul symbol is to express a state of non-signal, and for example, in the wireless 1394 system (ARIB STD-T72) standardized by the Association of Radio Industries and Businesses, there exists a nul symbol at the head of the OFDM packet. According to this system, since the DC offset is always estimated by using the nul symbol included in the packet, the DC offset value suitable every packet can be obtained. The DC offset estimated by the DC offset estimating unit is supplied to a DC offset correcting unit so as to adjust a reference voltage of the A/D converter and correct the DC offset voltage. After finishing the correction of the DC offset, the signal supplied from the A/D converter passes through the DC offset estimating unit and enters the OFDM demodulator, and the input OFDM signal is demodulated by the OFDM demodulator, thereby to restore the data transmitted from the OFDM transmitter.